But is it always true? And do the rail companies deserve the green plaudits they shower on themselves?

All these claims are based on average emissions for taking one passenger for one kilometre. In theory they take account of everything from how the power is generated to the type of engine and how full the plane or train usually is. And they are independently audited.

So we can take it on trust that one of Virgin's smart new Pendolinos, travelling half-full up the west coast main line, clocks up 27g per passenger kilometre. That compares with just over 150g for a typical, well-filled short-haul plane, and 180g for a car containing 1.2 passengers (the average occupancy). Likewise, Eurostar takes you to Paris for 11g/km or Brussels for 24g.

Eurostar does so well because its trains are mainly powered by French nuclear power stations. The company can't quite bring itself to say "our trains are greener because they run on nuclear power", but that is what it means.

Delve further into the data and it turns out that not all Virgin trains are anything like as clean as the Pendolinos, whose green credentials the firm advertises. Catch its most modern diesel train, the Voyager, and it emits 74g per passenger kilometre when travelling half-full – almost three times as much as the Pendolino.

Suddenly, that 78% claim has shrunk. In fact, if you catch a Voyager when it is just a quarter full – and I've been on plenty of those – then your emissions per kilometre travelled are about the same as sitting in a fullish plane. More leg room, but no greener.

Other companies have different figures for their average emissions. Scotrail, which runs the sleeper train to Euston, claims an average of 60g per passenger kilometre. Other companies like Southern and SouthWest Trains also quote the same figure, which turns out to be the government estimate of the national average. But averages are just that.

I used to take the overnight sleeper whenever I went to Scotland. I felt I was doing the right thing for the environment. In fact, I could be quite rude to people who flew to the same event.

But it looks like I was wrong. That Scotrail figure of 60g is for regular train carriages with 70-odd seats. It doesn't publish detailed stats but I am guessing that, like Virgin, they assume their carriages are about half-full. So maybe 40 passengers share the emissions for pulling a carriage from London to Glasgow.

But sleepers are different. You can't pack in as many beds as seats. And most people these days travel in single-berth compartments. If we assume there are 12 people snoozing the journey away in a typical sleeper carriage from Euston to Glasgow, that works out at 200g per passenger kilometre – rather more than the 150g for flying. Ouch.

This may be a bit of a special case. I am not saying that it is always, or even usually, better to fly. I am certainly not saying we should build a third runway at Heathrow to keep everybody off the trains.

What I am saying is that trains have had an easy ride so far over their emissions. One of the lines I regularly travel on is the Arun Valley line through West Sussex. It recently had a voltage upgrade to run new heavier trains with faster acceleration, air conditioning and power doors, which meant they needed twice the power of the old trains. No airline would ever contemplate doling that.

The Department for Transport admits that till recently, nobody on the railways thought much about greenhouse gases. "The increase weight of recent train designs… has demonstrated that too little emphasis has been placed on environmental issues in the past," officials wrote in the department's document, Rail Contribution to the Energy Review. Now the company is talking to rail operators about using lighter rolling stock. And about increasing the use of "regenerative braking" on electric trains. That means capturing the energy generated during braking as hybrid cars do and returning it to the power system for use by another train. On commuter lines this could cut electricity use by a fifth.

It is also a scandal how little of the British rail network is electrified. That switch alone cuts emissions on the line by a quarter or more. Yet two-thirds of the network still runs dirty diesel trains.

Right now, the railways could do with a lot more effort to cut their emissions and rather less greenwash about how environmentally friendly they are.

• How many more green scams, cons and generous slices of wishful thinking are out there? Please email your examples of greenwash to greenwash@theguardian.com or add your comments below